The present invention relates to a nonvolatile semiconductor memory device and a method for fabricating the same, and more particularly, it relates to a MONOS (metal-oxide-nitride-oxide-semiconductor) nonvolatile semiconductor memory device and a method for fabricating the same.
In accordance with recent increase of the degree of integration and cost reduction of nonvolatile semiconductor memory devices, a MONOS memory technique in which a virtual ground type array is provided for locally trapping charge has been proposed.
A MONOS memory device has a problem that electrons are trapped by a charge trapping film owing to UV generated during fabrication process so as to disadvantageously vary the threshold voltage. Therefore, it is significant to prevent the UV generated during the fabrication process from irradiating the MONOS memory device. The UV generated during the fabrication process is, for example, plasma emission employed principally in depositing an insulating film or the like or employed in various dry etching. For example, when a carbon fluoride gas such as a C4F8 gas is used in dry etching performed for forming interconnections, an emission spectrum having a peak in the vicinity of a wavelength of 250 nm is caused, and it is reported that the threshold voltage of a MONOS memory device is varied if the MONOS memory device is fabricated without blocking this light.
Now, conventional nonvolatile semiconductor memory devices and methods for fabricating the same will be described with reference to accompanying drawings.